Earth's atmosphere is composed of air.
Air is a mixture of gases, 78%
nitrogen and 21% oxygen with traces of water vapor, carbon dioxide,
argon, and various other components. We usually model air as a
uniform (no variation or fluctuation) gas with properties that
are averaged from all the individual components.
Any gas has certain
properties
that we can detect with our senses.
The values and relations of the properties define the
state
of the gas.

On this slide you will find typical values of the
properties of air at sea level static conditions for a
standard day. We are all aware that pressure and temperature
of the air depend on your location on the earth and the
season of the year. And while it is hotter in some seasons than
others, pressure and temperature change day to day, hour to hour,
sometimes even minute to minute during severe weather. The values
presented on the slide are simply average values used by engineers to
design machines. That's why they are called standard values.
We also know that all of the state-of-the-gas variables will change
with altitude, which is why the typical values are given at sea
level, static conditions. Because the gravity of the Earth holds the
atmosphere to the surface, as altitude
increases, air density, pressure, and temperature (for lower
altitudes) decrease. At the edge of space, the density is almost zero. The
variation of the air from the standard can be very important since it
affects flow parameters like the speed of
sound.

A gas is composed
of a large number of molecules which are in constant, random
motion.
The sum of the mass of all the
molecules is equal to the mass of the gas. A gas occupies some
volume in three dimensional space. For a given pressure and
temperature, the
volume
depends directly on the amount of gas.
Since the mass and volume are directly related, we can express both
the mass and volume by a single variable.
When a gas is moving, it is
convenient to use the density of a gas,
which is the mass divided by the volume the gas occupies.
The sea level standard value of air density r is

r = 1.229 kilograms/cubic meters = .00237 slug/cubic feet

When working with a static or unmoving gas, it is more convenient
to use specific volume, which is the
volume divided by the mass.
The sea level standard value of specific volume v is

v = .814 cubic meters/kilogram = 422 cubic feet/slug

The pressure of a gas equals the
perpendicular
force exerted by the gas divided by the
surface area on which the force is exerted.
The sea level standard value of air pressure p is

p = 101.3 kilo Newtons/square meter = 14.7 pounds/square inch

The temperature
of a gas is a measure of the kinetic energy of the molecules of the gas.
The sea level standard value of air temperature T is

T = 15 degrees C = 59 degrees Fahrenheit

A gas can exert a tangential (shearing) force on a surface, which
acts like friction between solid surfaces. This "sticky" property of
the gas is called the
viscosity
and it plays a large role in aerodynamic
drag.
The sea level standard value of air viscosity mu is

The density (specific volume), pressure, and
temperature of a gas are related to each other through the equation
of state. The state of a gas can be changed by external
processes, and the reaction of the gas can be predicted using the
laws of thermodynamics. A fundamental
understanding of thermodynamics is very important in describing the
operation of propulsion systems.